The hydration of fat free mass (FFM) and extracellular (ECW) and intracellular water (ICW) compartments were studied in 30 obese premenopausal women before and after a 3-mo weight-reduction program and again after a 9-mo weight-maintenance program. Body fat was determined by a four-compartment model. Total body water and ECW were determined by deuterium dilution and bromide dilution, respectively. After the weight-reduction period, mean weight loss was 12.8 kg, and body fat was reduced on average by 10.9 kg. During weight maintenance, changes in body mass and body fat were not significant. Before weight reduction, mean ECW/ICW ratio was relatively high (0.78 +/- 0.10). During the the study, total body water and ICW did not change significantly. ECW did not change significantly after weight reduction, but 12 mo after the start ECW was significantly increased by 1 liter. The ECW/ICW ratio increased to 0.87 +/- 0.12 (month 12). The hydration of the FFM increased from 74 +/- 1 to 77 +/- 2% during the weight reduction and remained elevated during weight maintenance. In conclusion, the ECW/ICW ratio and the hydration of the FFM, did not normalize during weight reduction and weight maintenance.